The invention relates to an acoustic monitor assembly particularly for use in a biosensor. Many different microbial cells are present throughout the environment, however it is often an onerous task to find out exactly where they are located and how many are present. Unfortunately, no hand-held instrument can provide this information even though exposure to certain microbial cells can have debilitating or fatal consequences. Because of these unsatisfactory consequences, there has arisen a strong interest in the monitoring of clinical fluids such as serum, saliva, urine and stools. In addition, the screening of drinking water contaminated with vibrio cholerae, cryptosporidium, total coliforms, legionella, Giardia and heterotrophic bacteria is also important.
Because any screening procedure has to allow for many different types of cells, as well as a great numbers of samples, the favored monitoring/screening approach would have to supply the answer quickly and at minimum cost. Therefore, the ideal candidate, based on current technology would be a synthesis between biological and electronic components, known more generally as a biosensor. Biosensors combine today's modern electronics with a very thin layer of antibodies, which react and adhere to the antigen, in this case particular epitope regions of the microbe cell surface. This adherent process localizes the cells where they can be detected with interrogating beams of light or sound. Unfortunately, the microbial cell counting problem, as described, has not yet yielded to the formidable biosensor revolution. Instead, there is a development restriction that is linked to biosensor systems becoming too complex and costly, betraying the original simplicity of the concept. Therefore, the realization of a new cell counting product requires the solution of two problems. First, there is the development problem, which requires for its solution the instigation of a new biosensor technology that is both cost-effective and simple to apply. The second, and main, problem is to apply the new biosensor technology to the construction of a handheld reader for microbial cell counts. A crucial characteristic required of this reader would be real-time on site monitoring for point-of-care and field diagnostics. currently, no product or technology is available to address these problems.
Conventional biosensors will now be described.
SPR--Surface Plasmon Resonance (Pharmacia) PA0 RN--Resonant Mirror (Fisons) PA0 PW--Piezoelectric Waveguide Devices PA0 PR--Piezoelectric Resonator Devices (Universal Sensors)
The SPR approach is relatively insensitive to the presence of cells. It is based on a glass prism coated with a metal layer. Laser light is projected onto the prism and at a particular angle generates what is known as a surface plasmon wave. This occurs at a specific angle whereby what is known as an evanescent wave "leaks" from the surface of the prism. Optical equipment must be configured carefully to record this angle as small fluctuations are crucial to the action of SPR. For example, protein binding to the prism surface elicits a biological perturbation, that modifies the angle. By monitoring the angle, all manner of surface processes can be monitored. Perturbation of the prism surface due to an accumulation of surface bound cells is possible to measure, however this detection scenario is rarely performed with SPR. This choice is a consequence of an interrogating light field interacting with only a small fraction in the cell volume. In addition, costs, specialized materials and the size of the overall system, make the SPR system generally inappropriate for cell measurements. A general sensing system is available from Pharmacia for $120,000.
This approach is slightly more sensitive to cells than the SPR system. Unfortunately, the same limitation of the SPR approach applies to the Resonant Mirror; interrogation of a very small fraction of the cell plus the need for specialized materials. A system supplied by Fisons is available in the $100,000 region.
Only research prototypes of the piezoelectric waveguide device are available. These work by projecting a high frequency (100 MHz or more) guided acoustic wave along a piezoelectric surface. The received acoustic wave is modified according to any surface viscosity or mass changes. This is a sensitive system so cells binding to the surface would cause an effect, however reported research systems prefer to focus on antibody measurements, demonstrating high sensitivity levels. This acoustic approach is less complicated than the optical approaches, SPR and RN, however cells remain difficult to measure due to limited interactions with the surface acoustic wave. Costs are typically $25,000 for research based systems.
The piezoelectric resonator devices are similar to the piezoelectric waveguide devices, sharing the same basic materials of construction, however they are simpler and operate at a far lower acoustic frequency (10 MHz). There are reports of cell measurements, however an additional step to dry the surface of the device is necessary. More recently, modifications that allow wet cell measurements have been initiated, with some success. Presently, no commercial device is available. Because of the larger interaction of the acoustic wave and the cell, more efficient cell detection is possible with this approach, in addition the system can be made portable. Fluctuations due to surface charge and temperature are the main system limitations. Universal Sensors supply a research system for $6500.
It is clear that although these biosensor technologies achieve useful subnanogram detection levels, substantial adaptations are required to address the demands of the counting problem outlined. In addition, the high costs of purchasing these systems coupled with their physical bulk restricts their application to bench mounted laboratory instrumentation.